staging: comedi: amplc_pci224: Replace NULLFUNC

The source code defines a macro NULLFUNC

  #define NULLFUNC 0

and uses it as a generic null function pointer constant.  This is
superfluous.  Just use NULL instead.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/amplc_pci224.c

@@ -265,9 +265,6 @@ Passing a zero for an option is the same as leaving it unspecified.
 /* Combine old and new bits. */
 #define COMBINE(old, new, mask)	(((old) & ~(mask)) | ((new) & (mask)))
 
-/* A generic null function pointer value.  */
-#define NULLFUNC	0
-
 /* Current CPU.  XXX should this be hard_smp_processor_id()? */
 #define THISCPU		smp_processor_id()
 
@@ -703,7 +700,7 @@ pci224_ao_inttrig_start(struct comedi_device *dev, struct comedi_subdevice *s,
 	if (trignum != 0)
 		return -EINVAL;
 
-	s->async->inttrig = NULLFUNC;
+	s->async->inttrig = NULL;
 	pci224_ao_start(dev, s);
 
 	return 1;